Conduit system, degassing, and flow dynamics of a rhyolite lava: A case study of the Shiroyama lava on Himeshima Island, Japan

Abstract

This study presents a description of a rhyolite lava-forming eruption, including the conduit system, degassing history during the lava flow dynamics. We examined the Pleistocene Shiroyama rhyolite lava on Himeshima Island, Japan. The lava is mainly characterized by locally developed obsidian. Based on the structural variation, the obsidian lithofacies correspond to the shallow conduit. The geological investigation and FTIR analyses showed that gas removal from the conduit magma proceeded via vesiculation, fracturing, and brecciation, allowing formation of the dense obsidian. Since the lava originally maintained some extent of water, the lava effervesced just after the effusion. This vesiculation resulted in pervasive bubble coalescence and the formation of abundant permeable pathways. The volcanic gasses escaped via those pathways, allowing collapse of the bubbles and deflation of the lava. AMS (anisotropy of magnetic susceptibility) results indicate that the lava spread concentrically.